Apparatus of this kind for extruding plastic material is known. It is used where the requirements of a particular duration of treatment of the material in the extruder and a good degassing cannot be attained in a single screw extruder. The two extruders are assembled in an L-form. A corresponding right angle arrangement for three extruders is also known. Although with this extruder arrangement the material to be extruded can be processed in the extruder for the required time and although the extruded material has at first satisfactory homogeneity, it has been observed that a deterioration occurs in the course of continued operation. In particular, it has been found that the extruded material contains small peices of material which have had a much longer residence in the extruder so that these material pieces disturb the homogeneity of the extruded material.
The present invention is based on the recognition that these small pieces of material with a high dwell time are particles which originate at the degassing position through the bursting of gas bubbles in the material to be extruded. These particles are deposited on the not fully filled screw in the degassing zone and can remain here a long time when they are carried along the vacuum at the degassing position in the direction of the vacuum connection. For in this zone between the degassing position and the vacuum connection there is a screw portion which is cut so as to act in the extrusion direction but which is not reached by the material stream as this is taken by the screw directly at the feed position. In the zone between the feeding position and the vacuum connection the material deposited on the screw remains there until the amount of material has become so great that it engages the inner wall of the cylinder and is thereupon propelled by the screw. However, it is a long time before sufficient material has been deposited on this part of the screw that a propulsion of the material is attained. As the time span between the depositing of the material and its propulsion by the screw is great, the collected material remains a long time on the screw. During this time the material undergoes a change under the temperature prevailing at this position which degrades it with respect to the main material stream. If this part with its altered characteristics is then advanced into the material stream, a non-homogeneous extrudant will result.